At a guess, the PNG contains metadata that Numbers would like to roundtrip back into itself. PNGs can be used to store compressed data reasonably easily. This is used quite a lot in the demo scene where you have hard limits on how big your program can be. For more information see JsExe: pouet.net/prod.php?which=59298 . I don’t have a copy of Numbers to test unfortunately.
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RobinAug 18 '14 at 12:27

3

I've tried deleting the image, and well, that doesn't affect the spreadsheet. And that's a little more odder...
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NibAug 18 '14 at 12:41

It doesn't happen for me. I get a roughly 100 KiB file. Half of it are the preview images and a third is ThemeStylesheet.iwa. I'm using Numbers 3.2 (1861).
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LukasAug 18 '14 at 12:44

tl;dr: it's the default shape fill, strictly unnecessary if there are no shapes.

XLSX, DOCX and other formats use OPC (the open packaging conventions), which mandates the zip container and describes how files should be laid out. If there is a file that you do not recognize, look in the various relationship files (they end in .rels).

In this case, the relevant line can be found in the themes relationship file xl/theme/_rels/theme1.xml.rels:

Keynote files exported to PowerPoint do things like this too. That PNG is a background fill from Keynote, and is made available to PowerPoint to be used as the default shape fill for shapes created in the exported document. Keynote not only exports your document, but also the template style elements it was created with - even if some of those style elements (i.e. the background image) haven't been used yet.